Method for illuminating liquid crystal display device, a back-light assembly for performing the same, and a liquid crystal display device using the same

ABSTRACT

A method for illuminating a liquid crystal display device, a back-light assembly for performing the same and a liquid crystal display device using the same can improve the front luminance and production yield. Lights from a lamp is incident to a LCD panel side, which are then primarily diffused for a uniformity of the luminous flux of the incident lights. The luminous flux of the primarily diffused lights is condensed with respect to a first orientation component on a plane in parallel with the LCD panel, and then partially and secondarily diffused for extending a view angle of pixels of the LCD panel. After the luminous flux of the secondarily diffused lights is condensed with respect to a second orientation component perpendicular to the first orientation on the parallel plane, the condensed lights are illuminated onto the LCD panel. A diffusion sheet for secondarily diffusing the luminous flux and a condensing sheet for condensing the second orientation component are formed by a single sheet to reduce the manufacturing process of the liquid crystal display device. Also, a defect produced from respective sheets is minimized to enhance the yield.

CROSS REFERENCE TO PRIOR APPLICATIONS

This application is a Continuation Application from a co-pending U.S.patent application Ser. No. 09/874,316 filed on Jun. 6, 2001 and issuedas U.S. Pat. No. 6,788,359, which is herein specifically incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for illuminating in a liquidcrystal display device, a back-light assembly for performing the sameand a liquid crystal display device using the same, and moreparticularly, to a method for illuminating a liquid crystal displaydevice and a liquid crystal display device using the same. The methodfor illuminating a liquid crystal display device enhances the frontluminance of the liquid crystal display device, reduces themanufacturing process steps and renders a thin and light liquid crystaldisplay device.

2. Description of the Related Art

Generally, a liquid crystal display device applies a voltage to aspecific molecular arrangement of a liquid crystal to convert itsmolecular arrangement. Then, the liquid crystal display device convertsthe changes of the optical properties such as birefringence, opticallinearity, dichroism and optical scattering characteristics of liquidcrystal cells that emits light according to the molecular arrangement,and uses the modulation of the light of the liquid crystal cells.

The liquid crystal display device is largely sorted into a TN (TwistedNematic) type and a STN (Super-Twisted Nematic) type. The liquid crystaldisplay device is, according to the driving method, sorted into anactive matrix display type, which uses a switching device and a TNliquid crystal, and a passive matrix type, which uses an STN liquidcrystal.

A distinguishable difference of two types is that the active matrixdisplay type is a TFT-LCD that drives an LCD by using a TFT and that thepassive matrix display type does not have such a complicated circuitassociated with a transistor because it has no transistor.

Since the liquid crystal display device is a passive light elementincapable of emitting light itself, the images are displayed by using aback-light assembly attached to the rear plane of the liquid crystalpanel.

Recently, there are several structures developed for contriving the slimand light products so as to have the leading edge of thecompetitiveness. Especially, the light-weight becomes a more importantfactor in view of the adoption chiefly to a portable computer, etc.

In such a liquid crystal display device, the role and function of theback-light assembly have been considered as increasingly important,because the dimension, light efficiency and so on differ depending onthe back-light assembly structure to affect on overallmechanical/optical characteristics of the liquid crystal display device.

A structure of a general back-light assembly used in the liquid crystaldisplay device is disclosed in U.S. Pat. No. 5,467,208.

FIG. 1 schematically shows the foregoing back-light assembly of theconventional liquid crystal display device. FIG. 2 schematically showsan illuminating process via the back-light assembly of the conventionalliquid crystal display device.

Referring to FIGS. 1 and 2, the liquid crystal display device includes alamp 1 for generating lights, a light guide plate 4 for guiding thelights from the lamp 1, and a lamp housing 2 installed to the side planeof the light guide plate 4 surrounding the lamp 1.

The lamp 1 employs a cold cathode, and the lights generated from thelamp 1 are incident via the side plane of the light guide plate 4.

The lamp housing 2 is formed with a reflection plate 6 on the innersurface thereof to reflect the lights from the lamp 1 toward the sideplane of the light guide plate 4, thereby enhancing the efficiency ofthe lights from the lamp 1.

The light guide plate 4 is formed of a transparent material of plasticssuch as acrylic resin for forming a panel shape having a sloped lowerplane and a horizontal upper plane (alternatively, a sloped upper plane)to allow the lights generated from the lamp 1 to advance toward an LCDpanel (not shown) seated on the upper portion after passing through theupper plane of light guide plate 4. Accordingly, the lower plane oflight guide plate 4 is formed with various patterns such as a minutedotted pattern printed for changing the advancing direction of thelights from lamp 1 into toward the LCD panel side.

A reflection plate 6 is formed under the lower plane of light guideplate 4, and a diffusion sheet 8, a first prism sheet 10, a second prismsheet 12 and a protecting film 14 are sequentially stacked on the upperplane of light guide plate 4.

The reflection plate 6 again reflects the lights that are not reflectedby the minute dotted pattern in the lights which have been generatedfrom the lamp 1 to advance toward the lower plane of the light guideplate 4 toward the upper plane of the light guide plate 4, therebyimproving the consistency of the lights transmitted to the upper planeof the light guide plate 4 while decreasing the optical loss of thelights incident to the LCD panel. As described above, the light guideplate 4 and the reflection plate 6 guide the lights generated from thelamp 1 toward the upper plane of the light guide plate 4.

At this time, the lights passing through the upper plane of the lightguide plate 4 include the lights outgoing by being to be sloped atvarious angles as well as the lights outgoing perpendicularly to theupper plane thereof.

The diffusion plate 8 placed between the light guide plate 4 and thefirst prism sheet 10 scatters the incident lights from the light guideplate 4 to prevent the partial concentration of the lights.Additionally, the diffusion plate 8 decreases the sloped angle of thelights advancing toward the first prism sheet 10 with respect to thefirst prism sheet 10.

The first prism sheet 10 and the second prism sheet 12 are formed in amanner that triangular bar-shaped prisms are formed to respective upperplanes in a regular arrangement, and the prism arrangement of the firstprism sheet 10 and that of the second prism sheet 12 are provided tocross each other at a predetermined angle.

The first and second prism sheets 10 and 12 respectively serve forcondensing the lights diffused by the diffusion plate 8. By doing so,the vertical incidence with respect to the protecting film 14 of thelights having passed through the first and second prism sheets 10 and 12becomes most favorable.

For this reason, almost all lights passing through the first and secondprism sheets 10 and 12 advance perpendicularly, so that the luminancedistribution over the protecting film 14 is uniform.

The protecting film 14 formed on the second prism sheet 12 not onlyprotects the surface of second prism sheet 12 but also diffuses thelights for uniform distribution of lights. The LCD panel 15 is installedon the protecting film 14.

In other words, in the back-light assembly structure of the conventionalliquid crystal display device, as shown in FIG. 2, the lights aregenerated from the lamp to be incident to the LCD panel 15 placed on theprotecting film 14 of the back-light assembly via the path provided bythe diffusion, condensing, condensing then to diffusion.

However, when applying the illuminating method of the back-lightassembly, the above-mentioned conventional liquid crystal display devicerequires several sheets for condensing/diffusing the lights on/from theupper portion of the light guide plate to therefore complicate theassembling process. Furthermore, the plurality of sheets used for thedevice may render high probability of puckering to degrade reliabilityof products and increase the manufacturing cost.

In addition, in the conventional diffusion plate, a plurality of beadsare distributed over the front plane and rear plane of the same densityto distribute the lights incident from the light guide plate to the rearplane of the diffusion plate.

When constructing the diffusion plate having the aforesaid structure,the precision of condensing function conducted by the first and secondprism sheets stacked on the diffusion plate is degraded to lower thefront luminance when the lights incident from the lower portion of thediffusion plate are aslant.

Moreover, in the conventional liquid crystal display device, if theprotecting film is eliminated to decrease the number of sheets stackedin the back-light assembly, the lights condensed by the first and secondprism sheets having the triangularly-shaped section are concentratedlycondensed only to the direction that is nearly perpendicular to theplane of the LCD panel to be incident to the LCD panel. The lightscondensed as described above cause interference with the pixels of theLCD panel, which in turn incites a moiré phenomenon of producing wavepattern throughout the screen.

SUMMARY OF THE INVENTION

Therefore, one object of the present invention is to provide a methodfor illuminating in a liquid crystal display device capable of improvingfront luminance by modulating a lights processing system in a method forilluminating in a liquid crystal display device.

Another object of the present invention is to provide a back-lightassembly of a liquid crystal display device and a liquid crystal displaydevice using the same capable of decreasing a manufacturing process andmanufacturing a slim and light product by modulating the lightsprocessing in the method for illuminating the liquid crystal displaydevice.

Still another object of the present invention is to provide a back-lightassembly of a liquid crystal display device capable of reducing thenumber of sheets employed therein while preventing a moiré phenomenon.

To achieve the above objects of the present invention, there is provideda method for illuminating in a liquid crystal display device performedby allowing lights generated from a light source to be incident to alight guide plate, and then primarily diffusing the incident lights fora uniformity of the luminous flux distribution. Thereafter, the luminousflux of the primarily diffused lights is primarily condensed withrespect to a first orientation component on a plane in parallel with anLCD panel, and the luminous flux of the lights condensed to have thefirst orientation for extending a view angle of pixels of the LCD panelis secondarily and partially diffused. The luminous flux of thesecondarily diffused lights is secondly condensed with respect to asecond orientation component that is perpendicular to the firstorientation on the parallel plane, and the secondarily condensed lightsare illuminated onto the LCD panel.

To achieve the above objects of the present invention, a back-lightassembly of a liquid crystal display device includes a light source forgenerating lights and a light guide plate formed to one side of thelight source for allowing the lights generated from the light source tobe uniformly incident to an LCD panel side. A first diffusion unitprovided to the LCD panel side of the light guide plate is formed toallow the reflectivity of the lights incident from the light guide plateto be higher than the light transmission, so that the incident lightsare primarily diffused for uniformly distributing the luminous flux ofthe incident lights. Also included parts are a first condensing unitprovided to the LCD panel side of the first diffusion unit for primarilycondensing the luminous flux of the primarily diffused lights withrespect to a first orientation component on a plane in parallel with theLCD panel, and a second diffusion unit for partially and secondarilydiffusing the luminous flux of the lights condensed to have the firstorientation to increase a view angle of pixels of the LCD panel. Inaddition to these, a second condensing unit secondarily condenses theluminous flux of the lights condensed to have the first orientation withrespect to a second orientation perpendicular to the first orientationon the parallel plane.

According to the present invention, the method for illuminating in theliquid crystal display device is improved to make it possible to embodythe liquid crystal display device having a high front luminance andreducing the number of sheets which form the back-light assembly of theliquid crystal display device. Therefore, overall thickness and weightof the liquid crystal display device can be decreased.

Further to these, the number of sheets used is reduced to decrease themanufacturing process, which in turn reduces the cost, and the overallquality of back-light assembly to contrive the higher product yield.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other objects and advantages of the presentinvention will become readily apparent with reference to the followingdetailed description, when considered in conjunction with theaccompanying drawings wherein:

FIG. 1 is a perspective view schematically showing a back-light assemblyof a conventional liquid crystal display device;

FIG. 2 is a view schematically showing an illuminating path via theback-light assembly of the conventional liquid crystal display device;

FIG. 3 is a perspective view schematically showing a back-light assemblyof a liquid crystal display device according to one embodiment of thepresent invention;

FIG. 4 is a view schematically showing an illuminating process via theback-light assembly of the liquid crystal display device according toone embodiment of the present invention;

FIG. 5 is a view schematically showing a diffusion sheet formation ofthe back-light assembly and an illuminating process thereof according toanother embodiment of the present invention;

FIG. 6 shows graphs plotting a result of measuring the luminanceaccording to view angles of the diffusion sheet of the conventionalliquid crystal display device and of that according to the presentinvention;

FIG. 7 is a schematic view for illustrating a back-light assemblystructure of the liquid crystal display device according to stillanother embodiment of the present invention;

FIG. 8 is a section view for illustrating a high condensing sheet of theliquid crystal display device according to still another embodiment ofthe present invention;

FIG. 9 is a section view for illustrating a high condensing sheet of theliquid crystal display device according to further another embodiment ofthe present invention; and

FIG. 10 is a view schematically showing an illuminating path formed bythe back-light assembly of the liquid crystal display device accordingto further another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, with reference to the accompanying drawings, a method forilluminating a liquid crystal display device, a back-light assembly anda liquid crystal display device using the method will be described indetail.

FIG. 3 is a view schematically showing a structure of a back-lightassembly of the liquid crystal display device according to oneembodiment of the present invention. FIG. 4 shows an illuminating pathformed by the back-light assembly of the liquid crystal display deviceaccording to one embodiment of the present invention.

Referring to FIGS. 3 and 4, the back-light assembly of the liquidcrystal display device according to the present invention includes alamp unit 23 and a light guide unit 35.

The lamp unit 23 includes a lamp 20 for generating lights and a lampreflection plate 22 for surrounding the lamp 20.

The lamp 20 usually employs a cold cathode, and the lights generatedfrom the lamp 20 is incident to a light guide plate 24 of the lightguide unit 35. The lamp reflection plate 22 serves for reflecting thelights generated from the lamp 20 to the light guide plate 24.

The light guide unit 35 is provided with the light guide plate 24 havinga reflection plate 26 at its lower surface. A first diffusion sheet 28is provided onto the light guide plate 24, i.e., onto the LCD panel sideof the light guide plate 24. A first condensing sheet 30, a seconddiffusion sheet 32 and a second condensing sheet 34 are sequentiallystacked on the first diffusion sheet 28 in the direction toward a LCDpanel 37.

The LCD panel 37 is seated on the upper portion of the light guide unit35.

The light guide plate 24 is formed of a transparent material of plasticgroup such as an acrylic so as to form a panel shape having a slantedlower plane and a horizontal upper plane. The light guide plate 24 maybe formed to have a wedge shape or a planar shape if the lamp unit 23 isformed to both sides of the light guide plate 24.

The light guide plate 24 is formed with the lamp unit 23 at one sidethereof, so that the lights generated from the lamp 20 are incident tothe light guide plate 24 to advance to the first diffusion sheet 28sitting on the light guide plate 24.

The lower plane of the light guide plate 24 may be formed with a varietyof patterns such as dotted patterns (not shown) printed for reflectingthe lights generated from the lamp to make them incident toward thefirst diffusion sheet 28 stacked on the upper plane of the light guideplate 24.

The reflection plate 26 is formed to the lower portion of the lightguide plate 24, which serves for repeatedly reflecting the lightswithout being reflected toward the first diffusion sheet 28 by thedotted patterns of the light guide plate 24 to make them incident towardthe first diffusion sheet 28.

The lights having reflected by the dotted patterns of the light guideplate 24 and the reflection plate 26 to be incident to the firstdiffusion sheet 28 advance in a manner to form a prescribed angle rangewith respect to the horizontal plane of the first diffusion sheet 28.

The first diffusion sheet 28 stacked on light guide plate 24 makes theluminous flux distribution of the lights reflected from the dottedpatterns of the light guide plate 24 and the reflection plate 26consistent. The first diffusion sheet 28 is formed with beads at theplane adjacent to the light guide plate 24 and the plane adjacent to thefirst condensing sheet 30.

Subsequently, the first condensing sheet 30 is stacked on the firstdiffusion sheet 28. It condenses the luminous flux of the lightsdiffused by the first diffusion sheet 28 with respect to a firstorientation component on the plane in parallel with the LCD panel 37. Inassociation with the first condensing sheet 30, a plurality oftriangular first prisms 30 a are arranged on the upper plane of thefirst condensing sheet 30 in the first orientation. The first prisms 30a condenses the lights with respect to the first orientation component.More specifically, the first prisms 30 a are arranged in the X-axisdirection on the plane that is perpendicular to the advancing directionof the lights.

A second diffusion sheet 32 is stacked on the first condensing sheet 30.It partially and secondarily diffuses the luminous flux of the condensedlights from the first condensing sheet 30 so as to have the firstorientation, thereby extending the view angle of the pixels of the LCDpanel 37.

Also, the second diffusion sheet 32 prevents the moiré phenomenon.

That is, if the first condensing sheet 30 and the second condensingsheet 34 are sequentially stacked without the second diffusion sheet 32,the lights condensed by the first and second condensing sheets 30 and 34having the triangularly-shaped section are incident to the LCD panel 37by being concentratedly condensed in the direction almost perpendicularto the plane of the LCD panel at the vertex portions of the first andsecond condensing sheets 30 and 34. The lights condensed in this mannercause interference with the pixels of the LCD panel to produce the moiréphenomenon that involves the wave pattern throughout the screen.

Therefore, the second diffusion sheet 32 that diffuses theperpendicularly condensed lights is stacked on the first condensingsheet 30 to diffuse the lights concentratedly condensed at the vertexportion of the first condensing sheet 30. After this, the lights arecondensed by the second condensing sheet 34 in the directionperpendicular to the LCD panel direction to thereby prevent theforegoing moiré phenomenon.

The second condensing sheet 34 is stacked on the second diffusion sheet32 for condensing the luminous flux of the lights diffused by seconddiffusion sheet 32 with respect to a second orientation componentperpendicular to the first orientation on the horizontal plane.

The second condensing sheet 34 is constructed such that a plurality oftriangular second prisms 34 a are arranged in parallel with one anotheron the upper plane of the transparent sheet in a prescribed direction tocross the prism arrangement direction of the first condensing sheet 30at a predetermined angle. Preferably, the second prisms 34 a arearranged in the Y-axis direction that is perpendicular to the X-axisdirection on the plane, i.e., perpendicular to the advancing directionof the lights.

Accordingly, the second condensing sheet 34 condenses the luminous fluxof the lights secondarily condensed in the second diffusion sheet 32 tohave the second orientation that is perpendicular to the firstorientation on the parallel plane, and then the finally condensed lightsare incident to the LCD panel 37 to improve the front luminance over theconventional one.

At this time, the second diffusion sheet 32 and the second condensingsheet 34 may be formed in a single sheet.

That is, the optical path of the conventional liquid crystal displaydevice is formed from diffusion, condensing, condensing to diffusion.This made it impossible to manufacture a single sheet for performing thediffusion after condensing the lights. However, in the presentinvention, as shown in FIG. 4, the path of the lights incident to theLCD panel 37 is formed by the path from the diffusion, condensing,diffusion and then condensing to be capable of manufacturing a sheetconsisting of a single layer that performs the condensing after thediffusion.

Therefore, in the back-light assembly structure of the liquid crystaldisplay device, the number of sheets stacked on the light guide platecan be reduced from the conventional four-layered structure to thethree-layered structure. Thus, the manufacturing process is shortenedwhile minimizing the quality problem of respective sheets, therebyenhancing the manufacturing yield of the liquid crystal display device.

FIG. 5 is a view schematically showing the diffusion sheet formation ofthe back-light assembly and illuminating process according to anotherembodiment of the present invention. FIG. 6 is graphs plotting theresult of measuring the luminance according to the view angles of thediffusion sheets of the conventional liquid crystal display device andthose according to the present invention.

Referring to FIG. 5, the back-light assembly of the liquid crystaldisplay device according to the present invention includes a light guideplate 24 for permitting the lights generated from a lamp 20 to beuniformly incident to an LCD panel side, and a first diffusion sheet 28stacked to the LCD panel side of the light guide plate 24. Also includedas parts are a first condensing sheet 30 provided to the LCD panel sideof the light guide plate 24, a second diffusion sheet 32 provided to theLCD panel side of the first condensing sheet 30, and a second condensingsheet 34 provided to the LCD is panel side of the second diffusion sheet32.

A plane 28 b of the first diffusion sheet 28 adjacent to the light guideplate 24 and a plane 28 a adjacent to the first condensing sheet 30 arerespectively formed with beads 29 a and 29 b. Here, the beads 29 a and29 b are distributed to make a haze value of the plane 28 b of firstdiffusion sheet 28 adjacent to the light guide plate 24 higher than thatof the plane 28 a of the first diffusion sheet 28 adjacent to the firstcondensing sheet 30.

The haze value represents the degree of diffusive transmission of theincident lights, which is defined as:

In the above equation, the quantity of diffusive transmission denotesthe quantity of the diffused lights in the lights that are incident tobe transmitted, and the quantity of overall transmission denotes thequantity of the total lights that are incident to be transmitted.

Because the beads 29 a and 29 b of the first diffusion sheet 28 functionfor diffusing the lights, the haze value becomes high when the beads 29a and 29 b are distributed in the high density, or the haze value is lowwhen being distributed in the low density.

Additionally, in order to differentiate the haze values of the plane 28b of the first diffusion sheet 28 adjacent to the light guide plate 24and of the plane 28 a of the first diffusion sheet 28 adjacent to thefirst condensing sheet 30, the beads respectively having differentrefractive indices may be used.

In this way, if the haze value of the plane 28 a of the first diffusionsheet 28 adjacent to the first condensing sheet 30 is higher than thatof the plane 28 b of the first diffusion sheet 28 adjacent to the lightguide plate 24, the lights incident aslant from the light guide plate 24to the first diffusion sheet 28 while having the incidence angle nearlyperpendicular to the plane of the first diffusion sheet 28 isdiffusively reflected from the plane 28 a of the first diffusion sheet28 adjacent to the first condensing sheet 30 or the plane 28 b thereofadjacent to the light guide plate 24.

Among these, the lights reflected from the plane 28 a adjacent to thefirst condensing sheet 30 are, as indicated by the optical path C,subject to a recycle process of being diffusively reflected from theplane 28 a of the first diffusion sheet 28 adjacent to the firstcondensing sheet 30, and then again reflected from the plane 28 badjacent to the light guide plate 24 of the first diffusion sheet 28.While being subject to the recycle process, the lights from the firstdiffusion sheet 28 toward the first condensing sheet 30 provided to LCDpanel side, i.e., the angle changed to have a nearly right angle withrespect to the plane of the LCD panel 37, when outgoing from the firstdiffusion sheet 28 to the first condensing sheet 30.

Furthermore, the lights reflected from the plane 28 b of the firstdiffusion sheet 28 adjacent to the light guide plate 24 are, as denotedby the optical path D, reflected by the reflection plate 26 underlyingthe light guide plate 24 to be again incident at the angle near to beperpendicular to the plane of the first diffusion sheet 28, therebyoutgoing to the first condensing sheet 30 while having the angle withhigh front luminance.

In other words, conventionally the lights outgoing from the light guideplate are heretofore concentrated much more onto the side rather thanthe front, so that the condensing rate of the lights in the verticaldirection is degraded regardless of being the diffusing and condensingvia the diffusion sheet and condensing sheet. For this reason, the highfront luminance cannot be obtained.

However, according to the present invention, even if the lights outgoingfrom the light guide plate 24 is concentrated onto the side to beincident to have the angle almost perpendicular to the plane of thefirst diffusion sheet 30, the lights are outgoing to the firstcondensing sheet 30 in the direction nearly perpendicular to the planeof the LCD panel 37 via the recycle process of repeated diffusion andreflection of the lights within the first diffusion sheet 30. Itimproves the condensing rate at the first condensing sheet 30 andenhances the front luminance of the liquid crystal display device.

The graphs representing the difference of the front luminance resultingfrom the haze value of the plane 28 b of the first diffusion sheet 28adjacent to the light guide plate 24 and that of the plane 28 a of thefirst diffusion sheet 28 adjacent to the first condensing sheet 30 areplotted in FIG. 6.

In FIG. 6, an alphabet A denotes the result obtained by measuring thefront luminance with respect to the back-light assembly of the liquidcrystal display device using the conventional diffusion sheet which hasthe haze value of the plane 28 b of the first diffusion sheet 28adjacent to the light guide plate 24 similar to that of plane 28 a ofthe first diffusion sheet 28 adjacent to the first condensing sheet 30.An alphabet B denotes the result obtained by measuring the frontluminance with respect to the back-light assembly of the liquid crystaldisplay device using the diffusion sheet according to the presentinvention, in which the haze value of the plane 28 a of the firstdiffusion sheet 28 adjacent to the first condensing sheet 30 is higherthan that of the plane 28 b of the first diffusion sheet 28 adjacent tothe light guide plate 24.

As shown in FIG. 6, it can be noted that the liquid crystal displaydevice equipped with the diffusion sheet B using the repeated diffusionand reflection processes according to the present invention has higherfront luminance over the conventional diffusion sheet A.

Also, the LCD panel side of the first diffusion sheet 28 is sequentiallystacked with the first diffusion sheet 32 and the second condensingsheet 34. Otherwise, the second condensing sheet 34 may be firstlystacked to LCD panel side of the first diffusion sheet 28, and thesecond diffusion sheet 32 is then stacked to the LCD panel side of thesecond condensing sheet 34.

At this time, there is a problem of degrading the front luminance due tosmall quantity of light transmission toward the LCD panel 37 if the hazevalue of the second diffusion sheet 32 is high. For this fact, thesecond diffusion sheet 32 has the haze value generally lower than thatof the first diffusion sheet 28. Preferably, the light transmission rateat the second diffusion sheet 32 is higher than that at the firstdiffusion sheet 28 by about 20˜30%.

The first diffusion sheet 32 and the second condensing sheet 34 may beformed of a single sheet.

When the second diffusion sheet 32 and the second condensing sheet 34are constituted by the single sheet as stated above, the foregoing firstdiffusion sheet 28 according to the present invention having the highfront luminance is employed to be able to reduce the number of the sheetused in the back-light assembly of the liquid crystal display devicewhile maintaining the high front luminance.

The structure of the back-light assembly of the liquid crystal displaydevice formed by constituting the aforementioned second diffusion sheet32 and second condensing sheet 34 in a single sheet is illustrated inFIG. 7.

Hereinafter, the single sheet for performing the diffusion andcondensing is referred to as a high luminance condensing sheet 36.

FIG. 7 is a schematic view for illustrating the structure of the liquidcrystal display device according to the present invention. FIG. 8 is asectional view for illustrating the high luminance condensing sheet ofthe liquid crystal display device according to the present invention.

Referring to FIGS. 7 and 8, the liquid crystal display device accordingto the present invention is provided with a lamp 20 and a light guideplate 24 formed to one side of lamp 20. In addition to these, a firstdiffusion sheet 28, first condensing sheet 30, a high luminancecondensing sheet 36 and a LCD panel 37 are sequentially stacked on lightguide plate 24.

At the other side of the lamp 20, a lamp reflection plate 22 is formedfor reflecting the lights generated from the lamp 20 to the light guideplate 24 while surrounding the lamp 20. A reflection plate 26 is formedto the plane of the light guide plate 24 opposite to the plane formedwith the first diffusion sheet 28.

A high luminance condensing sheet 36 is formed by a transparent sheet 40having a predetermined thickness, a diffusion layer 42 formed at thelower plane of transparent sheet 40, and a condensing layer 44 formed atthe upper plane of transparent sheet 40.

The transparent sheet 40 is the main body of high luminance condensingsheet 36, which is formed of polyester. The diffusion layer 42 is formedby distributing acrylic beads 42 a over the polyester resin. Thediffusion layer 42 uniformly diffuses the lights primarily condensed bythe first condensing sheet 30.

Moreover, the diffusion layer 42 serves for preventing the moiréphenomenon that produces the wave pattern throughout the screen asdescribed above, which has the haze value lower than that of the firstdiffusion sheet 28.

In the diffusion layer 42, it is preferable that the content of acrylicbeads (beads made from acrylic resin) 42 a is approximately 10˜50percents by weight with respect to the total amount of the overalldiffusion layer.

When the content of the acrylic beads 42 a is lower than 10weight/percent, the diffusion rate is lowered to induce the moiréphenomenon. If the content of the acrylic beads 42 a exceeds 50 percentsby weight, the front luminance is degraded.

The condensing layer 44 is formed by the second prisms 44 a formed ofacrylic resin, which are arranged to cross perpendicularly to thearranging direction of the first prisms 30 a of the first condensingsheet 30 underlying the high luminance condensing sheet 36.

The condensing layer 44 allows the luminous flux of the secondarilydiffused lights from the diffusion layer 42 to have the secondorientation perpendicular to the first orientation of the plane inparallel with the LCD panel 37.

Preferably, the thickness of the high luminance condensing sheet 36 isapproximately 160 μm, the angle of the vertex of the prism section isapproximately 90°, and the interval between respective prisms (distancebetween the ridgelines of one prism and adjacent prism) is roughly 50μm.

Except for the high luminance condensing sheet 36, the lamp 20, the lampreflection plate 22, the first diffusion sheet 28 and the firstcondensing sheet 30 are formed to have the same structures as those ofthe above-described embodiments.

FIG. 9 is a sectional view for illustrating the high luminancecondensing sheet of the liquid crystal display device according tofurther another embodiment of the present invention

Referring to FIG. 9, the vertex portions 46 b of the prisms 46 a whichare the condensing layer 44 of the high luminance condensing sheet 36,i.e., the portions of being seated with the LCD panel 37 among theprisms 46 a, may be formed to be round.

The illuminating path via the back-light assembly of the liquid crystaldisplay device according to above-described further another embodimentof the present invention is as shown in FIG. 10.

Referring to FIG. 10, it can be noted that the lights incident to theLCD panel 37 after having passed through the high luminance condensingsheet 36 according to the present invention are condensed to be moreperpendicular to the plane of LCD panel 37 than the lights incident tothe LCD panel 15 after being diffused from the protecting film 14 in theback-light assembly of the conventional liquid crystal display device asshown in FIG. 2, thereby having the higher front luminance.

The lights diffused and condensed by the high luminance condensing sheet36 are incident to the LCD panel 37 seated on top of the high luminancecondensing sheet 36.

According to the present invention, the path of the lights that havebeen emitted from the back-light assembly of the liquid crystal displaydevice to be incident to the LCD panel is formed by diffusion,condensing, diffusion then to condensing. Therefore, the condensing rateof the lights incident to the LCD panel is improved to obtain a liquidcrystal display device with high front luminance.

At the same time, the layers for performing the diffusion and condensingof the lights are formed by the single sheet, so that the number of thesheets stacked on the light guide plate of the back-light assembly ofthe liquid crystal display can be decreased from the four-layeredstructure to the three-layered structure.

By decreasing the number of sheets forming the back-light assembly ofthe liquid crystal display device as described above, the manufacturingprocess is reduced and the defect of the back-light assembly caused bythe quality problem of the sheet can be minimized. Consequently, thecost is economized and manufacturing yield is remarkably improved.Further to these, overall thickness and weight of the liquid crystaldisplay device can be minimized to easily attain the light weight andslimness of the liquid crystal display device.

Additionally, the diffusion unit is provided between condensing sheetsto prevent the generation of the moiré phenomenon due to theinterference of the lights concentratedly condensed only in a certaindirection with the pixels of the LCD panel.

Also, the diffusion sheet for regulating the incidence angle of lightsof the light guide plate incident from the light guide plate at the LCDpanel side to have the high front luminance with respect to the plane ofthe LCD panel via the repeated diffusive reflection process is providedto improve the front luminance of the liquid crystal display device.

As stated above, preferred embodiments of the present invention areshown and described. Although the preferred embodiments of the presentinvention have been described, it is understood that the presentinvention should not be limited to these preferred embodiments butvarious changes and modifications can be made by one skilled in the artwithin the spirit and scope of the present invention as hereinafterclaimed.

1. A liquid crystal display (LCD), comprising: an LCD panel; a lightsource; a light guide plate adjoining the light source; a diffuserformed between the LCD panel and the light guide plate; a condenserformed between the LCD panel and the diffuser; and a high luminancecondensing sheet formed between the LCD panel and the condenser, whereinthe high luminance condensing sheet comprises a transparent sheet havinga predetermined thickness, a diffusion layer formed at the lower surfaceof the transparent sheet, and a condensing layer formed at the uppersurface of the transparent sheet.
 2. The LCD of claim 1, wherein thediffusion layer comprises a plurality of beads formed for uniformlydiffusing the lights incident from the condenser.
 3. The LCD of claim 1,wherein at least one of the condenser and the condensing layer arecomprised of a plurality of prisms arranged in parallel with one anotherin a direction for condensing the lights incident from the diffuser andthe diffusion layer.
 4. The LCD of claim 1, wherein at least one of thecondenser and the condensing layer are formed by arranging prisms tocross one another at a predetermined angle.
 5. The LCD of claim 1,wherein a vertex portion of the prisms of the condensing layer adjacentto the LCD panel are rounded.
 6. A liquid crystal display (LCD),comprising: an LCD panel; a light source; a light guide plate adjoiningthe light source; a diffuser formed between the LCD panel and the lightguide plate; and a condenser formed between the LCD panel and thediffuser, wherein the diffuser comprises a plurality of beads formed onat least a surface of the diffuser adjacent to the light guide plate anda surface of the diffuser adjacent to the condenser, and the pluralityof beads are distributed to have a higher density at the surface of thediffuser adjacent to the condenser than that at the surface of thediffuser adjacent to the light guide plate.
 7. A liquid crystal display(LCD), comprising: an LCD panel; a light source; a light guide plateadjoining the light source; a diffuser formed between the LCD panel andthe light guide plate; and a condenser formed between the LCD panel andthe diffuser, wherein the diffuser comprises a plurality of beads formedon a surface of the diffuser adjacent to the light guide platedistributed to have a higher density than a plurality of beads formed ona surface of the diffuser adjacent to the condenser so that the surfaceof the diffuser adjacent to the condenser has a haze value that islarger than that of the surface of the diffuser adjacent to the lightguide plate.
 8. A liquid crystal display (LCD), comprising: an LCDpanel; a light Source; a light guide plate adjoining the light source; adiffuser formed between the LCD panel and the light guide plate; acondenser formed between the LCD panel and the diffuser; and a highluminance condensing sheet formed between the LCD panel and thecondenser, wherein the diffuser comprises a plurality of beads formed ona surface of the diffuser adjacent to the light guide plate distributedto have a higher density than a plurality of beads formed on a surfaceof the diffuser adjacent to the condenser so that the surface of thediffuser adjacent to the condenser has a haze value that is larger thanthat of the surface of the diffuser adjacent to the light guide plate toallow the reflectivity of the light incident from the light guide plateto be higher than the light transmission so that the incident light isprimarily diffused for uniformly distributing the luminous flux of theincident light.